Many manufacturing facilities which use multistep processing of materials to produce products have a similar resource at two or more steps. Some resources are used as a part of the manufacturing process, while other resources, such as air, may be present and can affect the process, but are not intended to be used by the process. In the case of air, it may be used by one process step, but merely present at another. Whether the resource is used or not, it is often desirable to control impurities which can otherwise contaminate the results of the manufacturing process.
For example, the manufacture of semiconductors from silicon wafers is a multistep process that is sensitive to impurities in the ambient air. At some steps in the process, the ambient air is relied upon to oxidize the silicon. At other steps in the process, ambient air is not used. At still other steps in the process, the existence of any ambient air actually disrupts the process step. In these latter types of steps, the process is performed in a vacuum, the achievement of which is an expensive, time consuming step as wafers are loaded by an operator who cannot work in a vacuum into equipment which must create a vacuum to operate.
In semiconductor fabricating facilities, or "fabs," particulate matter is a concern, because it interferes with the photolithographic, implant, etch or other processes that are used to form extremely tiny patterns on the wafers which are used to create the circuitry on the semiconductor. Because the patterns formed onto the silicon are so tiny, such processes requires extreme precision, and dust or pollen could dramatically alter the patterns created, causing the failure of the resulting semiconductor product. As operators of the equipment enter the fab, they carry this particulate matter with them which they stir up in the air as they move about the fab.
To combat the particulate problems, the air in prior art fabs was controlled to eliminate particulate matter, a process for which fabs are often referred to as "cleanrooms." Referring now to FIG. 1, a conventional fab is shown. The work area of the fab 100 has processing machines 120B, 120C, 122B, 122C, 124B, 124C, 126B, 126C on which or in which semiconductors are manufactured. Outside air 108 is cleaned of most particulate matter and pumped into the fab 100 by make up air handler unit 104 via inlet 132. One purpose a make up air handler unit 104 serves is to replace air that is lost accidentally through leakage in the walls, floor and ceiling of the work area of the fab 100. Make up air handler unit also resupplies oxygen lost by the breathing of the operators in the work area of the fab 100 and by the operation of some of the processing equipment 120B, 120C, 122B, 122C, 124B, 124C, 126B, 126C in the work area of the fab 100. Make up air handler unit 104 also resupplies air lost purposely through outlet 136 to outside air 109. Air contaminated with particulate matter is allowed to escape to outside air 109 in order to replace it with cleaner air from make up air handler unit 104, a process which serves to clean the air in the work area of the fab 100. Additionally, make up air handler unit 104 serves to pressurize the work area of the fab 100 in order to keep dust from entering the fab 100 from holes and cracks in the walls, flow and ceiling. Outlet 136 may contain a pressure controlled valve or fan to help ensure this pressurizing process is controlled.
To further clean the air in the work area of the fab 100, recirculation air handling unit 102 recirculates and filters particles in the air in the work area of the fab 100 via inlet 130, outlet 134 and conduit or duct 110.
The purity of the air is a function of the volume of air in the fab 100, the purity of and volume of the air from make up air handler unit 104, the effectiveness of, purification ability of, and volume of the air through, the recirculation air handling unit 102 and the generation of contamination from processing equipment 120B, 120C, 122B, 122C, 124B, 124C, 126B, 126C and other contamination, such as that brought in by the operators. If all of these factors are held to a constant rate, the level of contaminants in the air resource will reach an equilibrium level. Reduction of contamination may be made by varying any of the factors.
Another form of impurity can affect the operation of the circuitry in a semiconductor. Molecular impurities, particularly some organic molecules, metal ions, salts and heavy metals can affect the circuitry by interfering with the behavior of the various layers of the semiconductor device, which affect the characteristics of the semiconductor device. As device sizes in a semiconductors are reduced, the disruption of the semiconductor by a stray molecule plays an increasing role in the failure rate of the semiconductors produced in the fab 100.
For example, processing equipment 120B, 122B, 124B, 126B may include equipment used to increase the oxidation layer of the silicon used to manufacture semiconductors. Silicon exposed to air forms a layer of oxidation on its outer surface which can protect the silicon against molecular impurities. Naturally, this layer does not have adequate thickness to protect the silicon against the environmentally available molecular impurities to which the semiconductor may be exposed. However, the oxidized silicon may be driven deeper into the silicon by heating it. When the new layer of oxidation forms on the outer surface, the result is a thickened layer of oxidation that is adequate to protect the semiconductor against molecular impurities. Unfortunately, any molecular contamination on the surface of the silicon will also be driven into the silicon during this heating process.
Thus, the steps performed by processing equipment 120B, 122B, 124B, 126B are among the most sensitive to chemical impurities, yet produce very low levels of molecular contamination, mostly from contaminants introduced by other processing steps or other sources.
The molecular impurities may be introduced in the fab from the operators entering the fab and from the outside air introduced into the fab. In addition, the gases and chemicals used within some of the processing equipment 120C, 122C, 124C, 126C used in the fab itself to process the semiconductors, piping systems used to deliver these gases and chemicals, maintenance activities or accidents can also introduce a source of impurities in the ambient air in the fab. This equipment 120C, 122C, 124C, 126C may include vapor deposition equipment, ion implantation equipment, etch and other equipment which are used to deposit materials ON, dope, clean or etch, the semiconductor wafer. Because these types of equipment 120C, 122C, 124C, 126C serve as a source of stray molecules to which other processing equipment 120B, 122B, 124B, 126B is most sensitive, the two types of processing equipment 120B, 122B, 124B, 126B, and 120C, 122C, 124C, 126C are incompatible.
While the recirculation air handling unit 102 and make up air handler unit 104 equipment were improved to help remove molecular impurities from the outside air 108 and from recirculated air from the work area of the fab 100, the processing equipment 120C, 122C, 124C, 126C which served as an internal source of impurities remained a significant source.
The impact of this problem has been reduced in conventional chip fabricating facilities by separating the incompatible equipment in a "bay" and a "chase". The bay is a separate area of the fab that is used for processing steps that are most sensitive to impurities in a resources, while the chase is used for processing steps that are least sensitive to impurities in the resource. Referring now to FIGS. 1 and 2, the bay 100B contains processing equipment 120B, 122B, 124B, 126B and the chase 100C contains processing equipment 120C, 122C, 124C, 126C.
To maintain the purity of the air in each area 100B, 100C, each has its own make up air handler unit 104B, 104C supplying outside air 108. The particulate and molecular contamination in the outside air is filtered by make up air handler units 104B, 104C before pumping into bay 100B via inlet 132B and into chase 100C via inlet 132C. Recirculation air handling unit 102B, 102C recirculates and may filter particulate contamination only or particulate and molecular contamination via inlets 130B, 130C, outlets 134B, 134C and conduits 110B, 110C, which operate as inlet 130, outlet 134 and conduit 110 described above with reference to FIG. 1. Because the air supply and recirculation air handling unit system in the bay 100B and chase 100C operate independently, the significant source of molecular impurities from processing equipment 120C, 122C, 124C, 126C in the chase 100C does not contaminate the air resource supplied to processing steps performed in the bay 100B.
Outlets 136B, 136C to outside air 109 may contain valves or fans as described above with reference to FIG. 1, and may contain additional filtration equipment to lower the contamination of the outside air 109. Although the outlets 136, 136B, 136C to outside air 109 may be located at an area distant from inlets 132, 132B, 132C to outside air 108, cross contamination may occur making such additional filtration desirable.
The two systems, one for the bay 100B and one for the chase 100C allow the bay 100B and the chase 100C to each attain an equilibrium level of particulate and molecular contaminants, similar to that described above with reference to FIG. 1 for particulate contaminants. Because the bay 100B and the chase 100C operate as independent systems, the equilibrium levels are independent of each other, except for cross contamination of outside air.
In some types of semiconductor processing equipment 120B, 120C both are part of the same machine, and processing equipment 120B is incompatible with processing equipment 120C as described above. In such case, the bay 100B and the chase 100C may be adjacent to one another, the machine 120B and 120C is mounted into the wall separating the bay 100 and the chase 100C with the portion 120B of the machine most sensitive to contamination, such as a load port, located in, or having an door opening to, the bay 100B and the less sensitive portion 120C of the machine is located in the chase 100C.
Reducing molecular and particulate contamination is an expensive process. Nevertheless, as device sizes in semiconductors are reduced, impurities in the air in the bay 100B must be further reduced to maintain the yields of chips undamaged by contamination. It is desirable to minimize the contamination of the air in the bay for a given cost of purification of the air resource.